1. Field of the Invention
The present invention relates to a Shallow Trench Isolation (STI) type semiconductor device and a method for manufacturing an STI type semiconductor device. More particularly, the present invention relates to a method for manufacturing an STI type semiconductor device wherein the interface between a silicon substrate and a device isolation layer has enhanced electrical characteristics so that a leakage current may be reduced.
2. Description of the Related Art
As is well known in the art, a dynamic random access memory (DRAM) having a memory cell comprising a transistor and a capacitor may be downsized to produce a higher degree of integration in the DRAM.
Accordingly, as device dimensions of the memory cell are reduced, the dimensions of active regions and the space therebetween are reduced as well. Also, isolation regions, which play an important role in preventing current leakage between two adjacent devices, become narrow. High integration of the devices may cause several problems. For example, during formation of a field oxide layer, a bird's beak may occur at the edge of the active region allowing current leakage in a gate oxide layer.
A conventional trench isolation structure has been proposed to overcome the above-mentioned problems and is widely used in highly integrated semiconductor memory devices. The conventional trench isolation structure includes a trench region formed in a silicon substrate on which a well region is formed with a depth sufficient for isolating adjacent devices. A trench area is etched using a trench etching mask, which is formed by the patterning of a pad oxide and a silicon nitride which were deposited over the silicon substrate.
The etch process comprises the steps of forming a pad oxide and a nitride layer on a silicon substrate, selectively etching the pad oxide and the nitride layer, and dry etching the silicon substrate by using the patterned pad oxide and nitride layer as an etching mask.
Continuing, the trench area in the silicon substrate on which a well region was formed is anisotropically etched. Usually, the silicon substrate, which was exposed by the anisotropic etching process, has many crystal defects caused by the anisotropic dry etching process. A thermal process, such as annealing, follows to heal these crystal defects. Therefore, a thermal oxide is formed on the silicon substrate, but many crystal defects may still exist between the thermal oxide layer and the silicon substrate. Defects in the silicon substrate move easily so that the morphology of the trench sidewall deteriorates and dislocations occur easily. Additionally, dangling bonds that exist in the crystal defect become trap sites of electrical charges, thereby causing a leakage current during memory cell device operation.